1. Field of the Invention
The present invention relates to a color filter substrate and a color liquid crystal display device using the same, which is equipped in television sets, computers, word processors, office automation machines, and the like.
2. Description of the Related Art
FIG. 15 shows one example of a conventional color liquid crystal display device.
This color liquid crystal display device comprises a pair of substrates oppositely disposed to sandwich a liquid crystal layer 12. One of the substrates is a color filter substrate which comprises a glass substrate 1, and a light-shading member (a black mask) 3, a color filter coloring layer 4 and a smoothing layer (an overcoat layer) 5 formed on the glass substrate 1. Formed thereon are a display electrode 6a and an alignment film 7a. On the other glass substrate 2, a display electrode 6b an insulating film 8 and an alignment film 7b are formed. Both substrates 1 and 2 are attached together through a sealing material 10 containing glass beads 9 arranged along the periphery of the substrates, and plastic beads 11 for controlling a cell gap which are arranged between the substrates to form the liquid crystal layer 12 having a liquid crystal sealed in a gap between the glass substrates 1 and 2 surrounded by the sealing material 10. A counter portion of both display electrodes 6a and 6b which is not shaded by the light-shading member is a liquid crystal-lighting region.
The above-described color filter substrate is manufactured as shown in for example FIGS. 16A to 16I which illustrate a case where a metal film is used as the black mask 3.
First, a metal (chromium, etc.) film 3a is vacuum-deposited on the glass substrate 1 by sputtering or the like at a thickness of approximately 1000 angstroms, as shown in FIG. 16A, and a resist 13 is coated thereon, as shown in FIG. 16B. Then, the resist 13 is exposed using a photomask 14, as shown in FIG. 16C, and developed to form a resist pattern 13a as shown in FIG. 16D. Thereafter, it is etched to form a black mask 3, as shown in FIG. 16E.
Then, a coloring layer 15 is formed, as shown in FIG. 16F. The coloring layer 15 is formed on the whole surface of the substrate by coating a pigment-containing organic ink, or transferring a pigment-containing photo-sensitive resin. Subsequently, the coloring layer 15 is exposed using a photomask 16, as shown in FIG. 16G, and developed to form a color filter coloring layer 4 (a coloring layer corresponding to an R (red) pixel in this figure) in a dot shape, as shown in FIG. 16H. Similarly, coloring layers corresponding to G (green) and B (blue) pixels are formed in a dot shape. Additionally, a printing method may be used which comprises printing a pigment-containing organic ink on the prescribed region of the substrate for forming a color filter coloring layer.
Thereafter, an overcoat layer 5 is formed by a spin coating method or a printing method so as to improve the smoothness of the surface of the color filter substrate and to ensure its adhesive force to the display electrodes, as shown in FIG. 16I.
A common method of improving the surface smoothness of the color filter substrate comprises mechanically polishing the surface of the color filter substrate, or ameliorating the leveling ability of the overcoat material, as disclosed in Japanese Laid-open Patent Publications Nos. 2-275903 and 3-246503. Also, it includes forming adjacent coloring layers in such a manner so as not to overlap with each other in the formation of the color filter coloring layers corresponding to each of R, G and B pixels in a dot shape, or forming a light-shading member between adjacent coloring layers as sandwich thereof both sides, for fear of the unevenness of a liquid crystal domain and a cell thickness as well as the generation of color mixture, and the like. That is, a gap (a concave portion) 24 is commonly created between the adjacent coloring layers 4, as shown in FIG. 18. In FIG. 18, a indicates a cell thickness difference, b indicates a liquid crystal-lighting display region, and c and d indicate a pixel center portion and a pixel edge portion of the liquid crystal display region, respectively.
When the above-described color filter coloring layer is formed via a process of exposing, developing, baking and the like, the UV light is refracted toward the unwanted portion in the exposure to the coloring layer 15 to form a light-leaking region e which creates an overexposure portion f, as shown in FIG. 17A. Thus, an over-exposure portion 14a remains in the etching, as shown in FIG. 17B. Then, the pixel edge portion including the over-exposure portion 14a is thermally deformed by baking to form a coloring layer having round edge portions 14b with a barrel-shaped section at the time of the completion of the baking, as shown in FIG. 17C. When a smoothing film is formed thereon, the barrel-shaped portions are levelled to create a further difference in level.
Also, when a color filter coloring layer is formed by a printing method, the section of the coloring layer is barrel-shaped due to the surface tension of the printed coloring layer as having round edge portions similar to that shown in FIG. 17C, as found in for example Japanese Laid-open Patent Publication No. 4-62504. Thus, a gap (a concave portion) 24 is created between the adjacent coloring layers 4, as shown in FIG. 18. Moreover, in this case, the edge portions are not straight, but disoriented.
In recent years, liquid crystal display devices which permit animation operations, highly precise displays such as SVGA, XGA, etc., and large-screen displays in accordance with the development of multimedia personal computers, and large-scale liquid crystal display devices which can be adapted for CRT-substituted desk top computers have been demanded in the market. In order to meet these demands, a liquid crystal display device, especially a color liquid crystal display device using a STN(super twisted nematic)-type liquid requires such characteristics as high contrast, high brightness, high speed response, high display quality, low power consumption, and the like, for example, such characteristics as a contrast of 30:1 or more and a response speed of 200 ms or less, and the like. In order to accomplish these demands, it is necessary that the steepness of the liquid crystal (hereinafter referred to as an .alpha. value) be improved for the high contrast and high brightness, that the surface of the color filter substrate be uniform for the high display quality, and that the consumed current of the back light due to the high brightness be reduced for low power consumption. Also, it is demanded that the surface smoothness of the color filter substrate as well as its sectional shape (barrel-shaped) be improved for the improvement of all of these.
However, the above-mentioned conventional color filter substrate suffers from the following problems in the improvement of the contrast. That is, as shown in FIG. 18, a gap (a concave portion) 24 exists between the color filter coloring layers 4, and allows an overcoat material to flow into the gap 24 in the formation of the overcoat layer 5 to not only provide insufficient surface smoothness of the color filter substrate, but create a barrel-shaped section. Thus, a cell thickness difference a is created in a liquid crystal lighting display region b sandwiched by the display electrodes 6a and 6b. Since the cell thickness in the pixel edge portion d is increased by a compared to that of the pixel center portion c, a difference occurs in the V(voltage)-T(transmittance) curve between the pixel edge portion d and the pixel center portion c, as shown in FIG. 19. For that reason, when the portion c is at a state of V.sub.off, the portion d is nearly at a state of T.sub.on which allows the light to pass through so that the contrast is not improved. Also, a method of improving the steepness (i.e., lowering the a value) of the liquid crystal has been proposed so as to improve the contrast, but there is such a problem in a STN-type liquid crystal that the reduced a value of the liquid crystal causes a number of uneven display to deteriorate the display quality. That is, when the behavior of the liquid crystal is steepened, it is susceptible to the surface condition. For that reason, there is a limit of lowering the .alpha. value of the liquid crystal, in the case where a cell thickness difference occurs due to an insufficient surface smoothness of the color filter substrate which is contacted with the liquid crystal as well as a poor sectional shape of the color filter substrate.
Also, it is necessary to increase T.sub.on in the V-T curve of FIG. 19 for improving the brightness, but T.sub.on is correlated with the contrast, and thus it is necessary to increase a distance between T.sub.on and T.sub.off which is a dynamic range for improving the brightness as the contrast is T.sub.on /T.sub.off. However, because a difference occurs in the V-T curve shown in FIG. 19 due to the cell thickness difference a between the pixel center portion c and the pixel edge portion d as discussed above, a substantial dynamic range will be lowered in composing the V-T curve so that the transmittance which is the brightness is not improved. Moreover, since the low power consumption is achieved by increasing the brightness above the intended level and reducing the tube current of the back light by the same level, the power consumption cannot be reduced without the improvement of the brightness.
Also, it is necessary to ameliorate the surface smoothness as well as sectional shape of the color filter substrate for the improvement of the response speed and the display quality. Particularly, since the cell thickness is reduced for the high response speed (for example, from 6 .mu.m to 5 .mu.m or 4 .mu.m), a ratio of the gap of the surface unevenness (such as the difference in level) to the entire cell thickness will be greater. Thus, it is necessary to further improve the surface smoothness as well as sectional shape of the color filter substrate.
Moreover, when the conventional color filter substrate as shown in FIG. 18 is mechanically polished to smooth its surface, the surface of the barrel-shaped portion can be smoothed to some extent, but the concave portion 24 cannot be improved. On the other hand, when the surface is polished too much, even the color filter coloring layer 4 varies in thickness which may cause the variation of its characteristics such as hue, and the like. Also, the improvement of the leveling ability of the overcoat material is less effective for the conventional color filter substrate as shown in FIG. 18, and it is difficult to accomplish the intended surface smoothness in the liquid crystal lighting display region which is within .+-.0.01 .mu.m.